Cytochrome P-450 is a central component of the aryl hydrocarbon hydroxylase (AHH) system. Regulation of AHH enzymatic activity has been carefully studied in vivo, as well as in cell culture using Buffalo rat liver (BRL) cells and other cell types. The accessibility of the BRL cell culture system to well-controlled experimental manipulation makes it ideal for studying the regulation of biosynthesis of the cytochromes P-450 and of other AHH components. There is no direct information on the regulation of synthesis, processing and turnover of AHH system polypeptides, nor is there direct information on the regulation of rates of transcription, processing and degradation of the mRNAs for AHH-system enzymes. Our objective is to use the BRL cell culture system to study regulation of the AHH system at these two levels.